Change conversion of NTP64 and Timestamp to/from String

[JEnoch]

As discussed in #15 in 0.7.0 and previous versions the formatting and conversion of NTP64 and Timestamp into a String uses the RFC3339 representation, suffering of rounding and thus loss of precision when converting the fraction of seconds part into nanoseconds.
The consequence was that conversion from/to String was not bijective (i.e. you might not get back the original value after a 2-ways conversion).

This PR changes the default conversion of NTP64 and Timestamp into a String, formatting the time as an unsigned integer in decimal format. This conversion is lossless and bijective.

For conversion to RFC3339 format, those methods are added:

• NTP64::to_string_rfc3339_lossy()
• NTP64::parse_rfc3339()
• formatting with the alternate flag (e.g.: println!("{:#}", my_ntp64); )
• Timestamp::to_string_rfc3339_lossy()
• Timestamp::parse_rfc3339()
• formatting with the alternate flag (e.g.: println!("{:#}", my_timestamp); )

Note that those new methods are not available in no_std context.

Closes #15